The UN atomic watchdog hit back Monday at US criticism of the Iran nuclear deal, insisting its inspections there are the world's toughest and that Tehran is sticking to the accord.

"The nuclear-related commitments undertaken by Iran under (the 2015 accord) are being implemented," International Atomic Energy Agency head Yukiya Amano told reporters.

"The verification regime in Iran is the most robust regime which is currently existing. We have increased the inspection days in Iran, we have increased inspector numbers... and the number of images has increased," he said in Vienna.

"From a verification point of view, it is a clear and significant gain."

US President Donald Trump has called the agreement between Iran and six major powers reducing its nuclear activities in exchange for sanctions relief "the worst deal ever negotiated".

Trump has to certify in mid-October whether he believes Iran is abiding by the nuclear deal and that sticking to it is vital to US national security interests.

If, as seems increasingly likely, Trump decides not to give the green light, Congress will then have 60 days to debate whether to re-impose sanctions on Iran.

Trump's UN envoy, Nikki Haley, has been particularly critical, saying it was wrong to strike a deal ignoring Iran's missile programme and support for "terrorist" groups.

Haley, who held talks with Amano in Vienna on August 23, has also said that the IAEA is not able to carry out "anytime, anywhere" inspections.

"How do we know Iran is complying with the deal, if inspectors are not allowed to look everywhere they should look?," she said in Washington on September 5.

In fact, the IAEA has conducted at least 400 inspections of sites in Iran and 25 so-called "complimentary access" visits -- snap inspections requested at short notice -- since the deal came into force in January 2016.

These visits fall under the "Additional Protocol" agreement between Iran and the IAEA, which Tehran has been provisionally applying since 2016.

The IAEA data on these inspections, revealed in its 2016 Safeguards Implementation Report seen by AFP, however does not specify whether these sites are military or civilian.

"We have already have many complimentary access (visits) in Iran after the Implementation Day (when the deal entered into force) and we will contune to have many complimentary access (visits) in Iran," Amano said on Monday.

Source: AFP